In a DC-DC converter of the related art in which a fuel cell, a solar cell, or the like, is used as an input source, as a means for achieving stabilization of input power by controlling an input voltage to be constant, the input voltage being lowered by an operation of an output impedance when power is supplied, a means shown in FIG. 7 has been employed in which, when input supply power Pin is less than output supply power Pout, an input voltage is made constant, so that an output voltage enters a drooping state, and in which, when the input supply power Pin is greater than output supply power Pout, the input voltage rises, so that the output voltage becomes constant (regarding a solar cell, see, for example, Patent Document 1).
However, in the case of employing such a means, when the input supply power Pin is greater than output supply power Pout, the input voltage rises. Accordingly, in particular, when a fuel cell is used as an input source, a problem occurs in that a rapid change in voltage environment results in deterioration of the cell because the fuel cell uses chemical reactions to generate power. In addition, an operating voltage of the fuel cell varies depending on factors such as a cell temperature and a chemical reaction. For example, the fuel cell has a feature in that the amount of power that can be supplied varies depending on a difference in cell temperature even at the same operating voltage.
Accordingly, to solve the above problem, the present inventor has invented the DC-DC converter shown in FIG. 8, which is configured so that, an optimized reference voltage is input from the outside, and an arbitrary value of voltage-control input for controlling an input voltage of the DC-DC converter and an output voltage of a fuel cell are input so that a control signal is output (see, for example, Patent Document 2).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-341699    Patent Document 2: Japanese Patent Application Publication No. 2003-388747